The process industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other fluid processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process temperature transmitter provides an output related to a sensed process temperature. The temperature transmitter output is generally communicated over a process communication loop to a control room, or other process device such that the process can be monitored and/or controlled. In order to monitor a process temperature, the temperature transmitter generally includes, or is coupled to, a temperature sensor, such as a resistance temperature device (RTD).
An RTD changes resistance in response to a change in temperature. By measuring the resistance of the RTD, temperature can be calculated. Such resistance measurement is generally accomplished by passing a known current through the RTD, and measuring the associated voltage developed across the RTD.
In typical temperature transmitters, RTD measurements employ an internal fixed-resistance resistor as a reference. The reference resistor is used to measure the current flowing through the measurement circuit in order to calculate the sensor resistance. This requires two measurement points or scans for every update to get a high-accuracy resistive measurement that corresponds to a sensor temperature. Each scan typically requires a settling time and is typically converted over multiple power line cycles corresponding to the set line rejection frequency. An average RTD update can take approximately 250 milliseconds for the measurement including internal offset measurements. This time is generally not very long if the temperature transmitter is coupled to a single temperature sensor. However, with more recent process temperature transmitters, such as the high-density temperature transmitter device sold under the trade designation Model 848T Fieldbus, available from Emerson Process Management, of Chanhassen, Minn., eight RTD sensors can be employed. Using eight RTD sensors will multiply the approximately 250 milliseconds by eight. This update rate can, in some circumstances, be too slow for user acceptance. While the update rate can be increased dramatically if the reference scans are omitted, the accuracy of the temperature measurements would be prohibitively degraded.
Providing a temperature transmitter that is able to sense an RTD resistance and provide a high quality temperature output without an increased scan time would represent an advance in process industry temperature transmitters. While this advance would apply to any process industry temperature transmitters, it would be particularly beneficial to temperature transmitters that are coupled to a relatively large number of RTDs.